The invention relates to a high-energy weldable soft magnetic steel with high toughness in the heat-affected zone of weld joints, high specific electric resistance to reduce eddy currents, aging resistance and weathering resistance as well as its use for part of magnetic suspension railways which absorb carrying, guiding or driving forces, in particular side guide rails.
During the welding of structural steels, a coarse-grained structure is produced in a narrow zone next to the melt line as a result of the thermal stress of the material, which impairs the toughness properties. The size of the grain and the width of the coarse-grain zone are influenced by the energy per unit length during welding. With the increase of the energy per unit length, the grain is increased in size and, as a result, the energy absorbed in notched bar impact work deteriorates. As on the one hand the economical aspects of the welding is increased with rising energy per unit length and on the other hand a high toughness of the heat-affected zone is desired for the security of the component, there is a high demand for steels which are weldable with high energy per unit length without any permitted loss of toughness in the heat-affected zone, "Thyssen Techn. Berichte" (Thyssen Technical Reports), Volume 1/85, pages 42 to 49.
During the production of fine-grain structural steels the influence of fine precipitations, which can impair the austenite grain growth have long been used. Nitrides, carbides and carbonitrides of niobium and titanium as well as aluminum nitrides prevent the growth of austenite grains by obstructing the grain boundary movement. In the case of thermal stress caused during the welding, most precipitations dissolve and thus become ineffective. Only titanium nitride remains stable even at temperatures up to over 1400.degree. C. The effect of titanium nitrides on the obstruction of the austenite grain growth depends on their quantity, size and distribution. The dispersion of titanium nitrides is influenced by the content of titanium and nitrogen, as well as by the cooling conditions of the steel after the casting. Fine titanium nitride precipitations with a particle size of less than 0.020 .mu.m originate at titanium contents of less than 0.03% and a titanium/nitrogen ratio of 2 to 3.4. Under this prerequisite, the most effective obstruction in the austenite grain growth during the welding is achieved.
Steels whose alloy content is adjusted to corrosion resistance and the magnetic properties cannot be welded with high energy per unit length without losses in toughness in the heat-affected zone. The present invention is therefore based on the object of providing a soft magnetic steel which, on the one hand, can be processed with high energy per unit length by high-energy welding without any loss in toughness and, on the other hand, fulfils the requirements concerning high specific electric resistance, resistance to aging and weathering.